High-strength aluminum alloy plate

ABSTRACT

Provided is an Al—Mg—Si aluminum alloy sheet having a specific chemical composition including a transition element. The microstructure of the sheet includes grains refined to have a smaller average grain size, and includes nanometer-level fine transition-element-dispersed particles. This allows the aluminum alloy sheet to have higher strength during work hardening in forming into a structural component and, synergistically with this, to have better bake hardenability. The resulting aluminum alloy sheet, when formed into an automobile structural component, offers higher strength after bake hardening without deterioration in formability, even after natural aging at room temperature.

TECHNICAL FIELD

The present invention relates to Al—Mg—Si aluminum alloy plates or sheets. As used herein, the term “aluminum alloy sheet” refers to a rolled sheet such as a hot-rolled sheet or a cold-rolled sheet, after subjected to heat treatments (temper; T4) such as solution treatment and quenching, but before formed into a structural component to be used. Hereinafter, “aluminum” is also simply referred to as “Al”.

BACKGROUND ART

Social demands for weight reduction of vehicles such as automobiles have become higher and higher in consideration typically of global environment. To meet these demands, aluminum alloy materials are increasingly applied as automobile materials instead of ferrous materials such as steel sheets, because the aluminum alloy materials have formability and bake hardening properties (bake hardenability) at excellent levels and have lighter weights. The “bake hardening” is hereinafter also referred to as “BH”.

Representative examples of aluminum alloy sheets for automobile large-sized panel components such as outer panels and inner panels include AA or JIS 6xxx-series aluminum alloy sheets, which are Al—Mg—Si alloy sheets. The AA or JIS 6xxx-series are hereinafter also simply referred to as “6xxx-series”. The 6xxx-series aluminum alloy sheets have chemical compositions essentially containing Si and Mg and have low yield strength (low strength) upon forming (shaping) to surely offer good formability. In addition, the 6xxx-series aluminum alloy sheets have excellent bake hardenability, in which they have higher yield strength (strength) by heating in artificial aging (hardening) treatments such as paint bake of panels, which are performed after forming, to surely offer required strength.

For further weight reduction of automobile bodies, demands are made to apply aluminum alloy materials to, of automobile components, not only the panel components, but also automobile structural components exemplified typically by frame components such as frames and pillars; and reinforcements such as bumper reinforcements and door beams.

These automobile structural components require still higher strength as compared with the automobile panels. To be applied to the frame components or reinforcements, the 6xxx-series aluminum alloy sheets, which have been applied to the automotive body panels, require still higher strength.

However, it is not so easy to achieve such higher strength of conventional 6xxx-series aluminum alloy sheets without significantly changing chemical compositions and production conditions of conventional equivalents and without adversely affecting other properties such as formability.

Various control techniques on size and number density of transition-element-dispersed particles have been proposed as microstructural control so as to allow 6xxx-series aluminum alloy sheets to be improved in properties, such as bake hardenability, which are required for the panel components.

For example, Patent Literature (PTL) 1 proposes a 6xxx-series aluminum alloy sheet for use as the panel components, where the aluminum alloy sheet has press formability and bendability at better levels by allowing dispersed particles each having a size of 0.5 μm or more to be present in an average number density of 3000 to 20000 per square millimeter.

In the working examples in the literature, this aluminum alloy sheet offers a 0.2% yield strength of at highest about 205 MPa, where, before measurement of the 0.2% yield strength, the sheet has been subjected sequentially to natural aging at room temperature for 3 month (90 days) after its production (after heat treatment (temper)), application with a 2% strain, and artificial aging at 170° C. for 20 minutes.

PTL 2 proposes an aluminum alloy sheet for use as the panel components, where the aluminum alloy sheet has press formability and hem bendability at excellent levels, by controlling dispersed particles to have an average diameter of 0.02 to 0.8 μm and to be present in an average number density of 1 or more per cubic micrometer so as to refine recrystallized grains to have a smaller average grain size of 45 μm or less. The aluminum alloy sheet proposed in PTL 2 also offers low strength in terms of 0.2% yield strength of at highest about 205 MPa, where, before the measurement of the 0.2% yield strength, the sheet has been subjected sequentially to natural aging at room temperature for 3 months (90 days) after its production, deep draw forming, and artificial aging at 180° C. for 20 minutes.

CITATION LIST Patent Literature

PTL 1: Japanese Unexamined Patent Application Publication (JP-A) No. 2007-169740

PTL 1: Japanese Patent No. 3802695

SUMMARY OF INVENTION Technical Problem

The conventional 6xxx-series aluminum alloy sheets are controlled on size and number density of transition-element-dispersed particles so as to be used for the panel components. These conventional 6xxx-series aluminum alloy sheets, however, are insufficient in strength when they are to be used for the frame components or reinforcements, which require high strength in terms of 0.2% yield strength of 250 MPa or more after artificial aging.

In contrast, attempts may be made to perform the artificial aging at a higher temperature of typically about 200° C. so as to offer higher strength. However, these attempts fail to allow the aluminum alloy sheet to have higher strength in terms of 0.2% yield strength of 250 MPa or more after artificial aging.

In addition, there are limitations and restrictions on the artificial aging (hardening) treatment temperature due typically to treatment efficiency, deterioration of baked paints, and reduction in strength caused by over-aging; and there are circumstances which impede achievement of such higher temperature.

The present invention has been made to solve these problems and has an object to provide a 6xxx-series aluminum alloy sheet which can be used for the structural components typically of automobiles and which can have higher strength without significantly changing chemical compositions and production conditions of conventional 6xxx-series aluminum alloy sheets.

Solution to Problem

To achieve the object, the present invention provides a high-strength aluminum alloy sheet as follows. This aluminum alloy sheet is an Al—Mg—Si aluminum alloy sheet including, in mass percent, Mg in a content of 0.3% to 1.5%, Si in a content of 0.3% to 1.5%, and at least one transition element selected from the group consisting of Mn in a content of 0.1% to 0.8%, Zr in a content of 0.04% to 0.20%, Cr in a content of 0.04% to 0.20%, and Sc in a content of 0.02% to 0.1%, with the remainder consisting of Al and unavoidable impurities. In a microstructure in a thickness central part of the sheet, the sheet has properties as follows. The sheet has an average grain size of 100 μm or less. Disposed particles containing the at least one transition element have an average equivalent circle diameter of 50 to 300 nm, as measured by TEM-EDX at 50000-fold magnification. Transition-element-dispersed particles each having an equivalent circle diameter of 20 to 400 nm are present in an average number density of 5 or more per cubic micrometer.

Advantageous Effects of Invention

In the present invention, how the transition-element-dispersed particles in the microstructure of a 6xxx-series aluminum alloy sheet affect the bake hardenability was reconsidered, on the precondition that conventional aluminum alloy chemical compositions and production conditions are not significantly changed.

As a result, it was found that the 6xxx-series aluminum alloy sheet can have significantly better bake hardenability when the aluminum alloy sheet contains nanometer-level fine transition-element-dispersed particles in a substantial amount (in a substantial number density) in the microstructure.

This is because the fine transition-element-dispersed particles contribute not only to higher yield strength (strength) obtained by heating in artificial aging (hardening), such as paint bake, of a structural component formed from the material sheet; but also to work hardenability during forming (shaping) of the material sheet into the structural component, where the forming is performed upstream from the artificial aging.

Specifically, the fine transition-element-dispersed particles contribute both to better work hardenability (strength) during forming of the material sheet and to higher yield strength (strength) through artificial aging of the structural component after forming, and, as a result of synergistic effects of them, specifically effectively contribute to better bake hardenability.

In addition, the fine transition-element-dispersed particles do not adversely affect the formability of the material sheet into a structural component.

With the present invention, a 6xxx-series aluminum alloy sheet is controlled to include the fine transition-element-dispersed particles which are present in a substantial amount, which have a specific average equivalent circle diameter, and which are present in a specific average number density, on the precondition that the 6xxx-series aluminum alloy sheet is controlled to have a smaller (finer) average grain size in the thickness central part. The 6xxx-series aluminum alloy sheet, as having this configuration, can have high strength necessary for automobile structural components such as frame components and reinforcements, without deterioration in formability, where the automobile structural components require higher strength as compared with conventional panel components.

DESCRIPTION OF EMBODIMENTS

Some embodiments of the present invention will be specifically illustrated about individual required conditions below.

Chemical Composition

Initially, the chemical composition of the Al—Mg—Si aluminum alloy sheet according to the present invention will be illustrated below. Hereinafter the Al—Mg—Si aluminum alloy sheet is also referred to as a “6xxx-series aluminum alloy sheet”. With the present invention, the Al—Mg—Si aluminum alloy sheet is allowed to have high strength satisfactory for use as the frame components or reinforcements, without significantly changing conventional chemical compositions and production conditions.

To achieve the object in terms of chemical composition, the 6xxx-series aluminum alloy sheet is controlled to have a chemical composition including, in mass percent, Mg in a content of 0.3% to 1.5%, Si in a content of 0.3% to 1.5%, and at least one transition element selected from the group consisting ofMn in a content of 0.1% to 0.8%, Zr in a content of 0.04% to 0.20%, Cr in a content of 0.04% to 0.20%, and Sc in a content of 0.02% to 0.1%, with the remainder consisting of Al and unavoidable impurities. All percentages as contents of elements are in mass percent.

The content ranges and their significance, or permissible levels of the elements in the 6xxx-series aluminum alloy will be described below.

Mg: 0.3% to 1.5%

Magnesium (Mg) forms aged precipitates with Si, offers age hardenability, and is essential for a yield strength necessary as structural components typically of automobiles, where the aged precipitates offer solid-solution strengthening and contribute to higher strength upon artificial aging such as paint bake treatment.

Mg, if present in a content less than 0.3%, causes the aluminum alloy sheet to have insufficient strength. In contrast, Mg, if present in a content greater than 1.5%, tends to cause shear zones to be formed during cold rolling, and this may cause cracking during rolling. For these reasons, the Mg content is controlled to be 0.3% or more, preferably 0.4% or more, and to be 1.5% or less, preferably 1.2% or less.

Si: 0.3% to 1.5%

Silicon (Si) also forms aged precipitates with Mg, offers age hardenability, and is essential to give strength (yield strength) necessary for structural components typically of automobiles, where the aged precipitates offer solid-solution strengthening and contribute to higher strength upon artificial aging such as paint bake treatment.

Si, if present in a content less than 0.3%, causes the aluminum alloy sheet to have insufficient strength. In contrast, Si, if present in a content greater than 1.5%, forms coarse compounds and causes the aluminum alloy sheet to have inferior ductility. For these reasons, the Si content is controlled to be 0.3% or more, preferably 0.7% or more and to be 1.5% or less, preferably 1.4% or less.

Mn, Zr, Cr, and Sc

The transition elements Mn, Zr, Cr, and Sc are principal elements constituting the transition-element-dispersed particles specified in the present invention. These elements form transition-element-dispersed particles in ingots and/or in a final sheet, thereby contribute to grain refinement, and contribute to higher strength.

The transition-element-dispersed particles, when refined to the nanometer-level as specified in the present invention, allow the aluminum alloy sheet to have significantly higher strength after BH. This is not only because the particles contribute to better work hardenability during forming of the material sheet into a structural component, where the forming is performed upstream from artificial aging; but also contribute to better artificial age hardenability (bake hardenability) of the structural component after forming.

Any of Mn, Zr, Cr, and Sc, if present in an excessively low content, causes the dispersed particles to be present in a lower number density and causes the aluminum alloy sheet to have lower work hardenability during forming into a structural component and to have lower artificial age hardenability (bake hardenability). Accordingly, the forming and artificial aging give a smaller increase in yield strength, with smaller magnitude of the synergistic effects. This causes the aluminum alloy sheet after artificial aging to fail to have sufficiently increased strength at a level necessary for a structural component.

In contrast, any of Mn, Zr, Cr, and Sc, if present in an excessively high content, tends to form coarse compounds, which act as fracture origins, and causes the aluminum alloy sheet to have ductility and/or strength at lower level contrarily.

For these reasons, the aluminum alloy sheet includes at least one element selected from the group consisting of Mn in a content of 0.1% to 0.8%, Zr in a content of 0.04% to 0.20%, Cr in a content of 0.04% to 0.20%, and Sc in a content of 0.02% to 0.1%.

Other Elements

For higher strength, the aluminum alloy sheet according to the present invention may further include at least one element selected from the group consisting of Cu in a content of greater than 0% to 0.5% (0.5% or less, excluding 0%), Ag in a content of 0.01% to 0.2%, and Sn in a content of 0.001% to 0.1%, in addition to the above-mentioned elements.

These elements, in common, effectively allow the sheet to have higher strength and are considered to be equieffective elements for higher strength. Naturally, however, specific mechanisms of the effects of these elements are partially identical and partially different.

Copper (Cu) contributes to higher strength due typically to solid-solution strengthening. Silver (Ag) is useful for better artificial age hardenability (bake hardenability) and effectively promotes precipitation of compound phases such as Guinier-Preston zone in grains in the sheet microstructure, by artificial aging performed at a relatively low temperature for a relatively short time. Tin (Sn) captures atomic vacancies, thereby restrains Mg and Si from diffusing at room temperature, and restrains the aluminum alloy sheet from having higher strength at room temperature (natural aging at room temperature). Upon artificial aging, this element releases the captured vacancies, promotes the diffusion of Mg and Si, and effectively allows the aluminum alloy sheet to have better bake hardenability.

However, any of these elements, if present in an excessively high content, tends typically to form coarse compounds to impede sheet production, and causes the resulting aluminum alloy sheet to be inferior not only in strength and bendability, but also in corrosion resistance. In particular, Cu, if present in an excessively high content, causes the aluminum alloy sheet to have significantly inferior bendability. For these reasons, each of these elements, when to be contained, is contained in a content equal to or less than the upper limit.

Impurities

The other elements such as Fe, V, Ti, B, and Zn are unavoidable impurities, which tend to be incorporated typically from scrap as a raw material to be melted to form ingots, and are preferably minimized. However, these elements may be present in contents within ranges allowable in standards such as Japanese Industrial Standards (JIS) in view typically of efficiency of melting and refining. For example, Fe may be contained in a content of preferably 0.5% or less.

Microstructure

After controlling the aluminum alloy sheet to have the 6xxx-series alloy chemical composition as described above, the present invention specifies the microstructure of the 6xxx-series aluminum alloy sheet. Specifically, after controlling the sheet to have smaller (finer) average grain size in the sheet thickness central part, the present invention controls the fine transition-element-dispersed particles to be present in a substantial amount and specifies both the average equivalent circle diameter and the average number density of the fine transition-element-dispersed particles.

These requirements (conditions) on microstructure are important, essential ways for the 6xxx-series aluminum alloy sheet to have better bake hardenability to thereby have higher strength, on the preconditions that conventional aluminum alloy chemical compositions and production conditions are not significantly changed and that the formability of the aluminum alloy sheet is not adversely affected.

Average Grain Size

The refinement (reduction) in average grain size in the thickness central part serves as a precondition for allowing the fine transition-element-dispersed particles to offer their effects. Specifically, the fine transition-element-dispersed particles offer the effects only after the grain microstructure in the thickness central part be a fine grain microstructure in terms of average grain size of 100 μm or less. If the grain microstructure in the thickness central part be a coarse grain microstructure in terms of average grain size of greater than 100 μm, the fine transition-element-dispersed particles offer reduced effects, for example, reduced in half. From that viewpoint, the grain refinement can be considered as a precondition for the fine transition-element-dispersed particles to offer the effects surely.

As used herein, the term “grain size” refers to a grain size in the thickness central part in the sheet rolling direction, of a longitudinal section in the sheet rolling direction, where the longitudinal section is a cross section cut along the sheet rolling direction. The average grain size is measured and evaluated by the line intercept method in the sheet rolling direction. Specifically, a sample is sampled from the longitudinal section in the thickness central part of a sheet after T4 heat treatments, but before forming into a structural component. The sample is mechanically polished on its surface, and electrolytically etched in a 5:400 mixture (solution) of tetrafluoroboric acid and water at a voltage of 30 V, a solution temperature of 20° C. to 30° C. for a time of 60 to 90 seconds. In consideration of variation in sheet materials, the average grain size is measured by visual observation in 10 view fields at arbitrary measurement points in the thickness central part, using a 50× optical microscope equipped with a polarizing plate, where five lines each having a line length of 500 μm were visually observed per view field.

Transition-Element-Dispersed Particles

On the precondition that the average grain size is controlled as above, the microstructure in the sheet thickness central part is specified as follows. In the microstructure, transition-element-dispersed particles have an average equivalent circle diameter of 50 to 300 nm as measured by TEM-EDX at 50000-fold magnification; and transition-element-dispersed particles each having an equivalent circle diameter of 20 to 400 nm are present in an average number density of 5 or more per cubic micrometer.

The 6xxx-series aluminum alloy sheet before artificial aging offers significantly better bake hardenability, when, in the sheet microstructure, the transition-element-dispersed particles are controlled to have an average equivalent circle diameter of 50 to 300 nm, and fine transition-element-dispersed particles each having an equivalent circle diameter of 20 to 400 nm are controlled to be present as much as possible, in terms of average number density of 5 or more per cubic micrometer, where the bake hardenability is increased with an increasing average number density of the transition-element-dispersed particles each having an equivalent circle diameter of 20 to 400 nm.

Although its mechanism has not yet been clarified, the transition-element-dispersed particles contribute to better bake hardenability probably because the transition-element-dispersed particles having the specific size and being present in the specific number density particularly contribute to better work hardenability upon application of prestrain, and contribute to restrainment of recovery of dislocations during heat treatment corresponding to paint bake treatment, where the dislocations have been introduced by the application of prestrain.

In addition, it is satisfactorily advantageous that the fine transition-element-dispersed particles do not adversely affect the formability of the material sheet into a structural component.

The transition-element-dispersed particles having an average equivalent circle diameter within the range, and the transition-element-dispersed particles each having an equivalent circle diameter within the range contribute not only to higher yield strength (strength) upon heating in artificial aging, but also to better work hardenability in the forming of the material sheet into an automobile structural component, where the forming is performed upstream from the artificial aging. Accordingly, the fine transition-element-dispersed particles contribute to better work hardenability in forming of the material sheet into an automobile structural component and to better artificial age hardenability of the automobile structural component after forming, and as a result of synergistic effects of them, contribute to significantly better bake hardenability.

The transition-element-dispersed particles, if having a small average equivalent circle diameter of less than 50 nm, or, conversely, if having a large average equivalent circle diameter of greater than 300 nm and being coarsened, o r none or lower synergistic effects for higher strength after BH.

The fine transition-element-dispersed particles each having an equivalent circle diameter of 20 to 400 nm, if being present in an average number density of less than 5 per cubic micrometer, fail to offer the synergistic effects because of an excessively small number (number density) of the fine transition-element-dispersed particles. The present invention specifies the average number density of transition-element-dispersed particles as measurable by TEM-EDX at 50000-fold magnification, with consideration given to the sizes (equivalent circle diameters) of the particles.

In view of production limitations, the upper limit of the average number density of the transition-element-dispersed particles each having an equivalent circle diameter of 20 to 400 nm is about 100 per cubic micrometer.

A lower average number density of such fine transition-element-dispersed particles is caused by an insufficient content of any of the transition elements to constitute the particles. However, the fine transition-element-dispersed particles may be present in a lower average number density also because formed transition-element-dispersed particles fail to grow and fail to have an equivalent circle diameter of 20 nm or greater (particles are excessively small), or conversely, the particles may coarsen and have an equivalent circle diameter of greater than 400 nm. These may occur due to conditions of the sheet production method, even when the transition element contents fall within the appropriate ranges.

Measurement on Transition-Element-Dispersed Particles

The average number density (number per cubic micrometer) of the transition-element-dispersed particles each having an equivalent circle diameter of 20 to 400 nm, as specified in the present invention, is measured using a transmission electron microscope (TEM; FE-TEM) at 50000-fold magnification, where the microscope has energy dispersive X-ray spectroscopic (EDX) function to identify (distinguish) the dispersed particles.

The sheet as a measurement object is an aluminum alloy sheet which is a rolled sheet such as a hot-rolled sheet or a cold-rolled sheet and which is a T4 sheet after heat treatments such as solution treatment and quenching, but before subjected to forming (such as bend forming) into a structural component to be used. The measurement object sheet is one before forming, because the transition-element-dispersed particles in this stage also affect the work hardenability.

Specifically, the measurement may be performed by a method as follows. A sample is sampled from the thickness central part of the T4 sheet before forming, and processed into a thin film sample for TEM. A photograph of the microstructure in the thickness central part is taken by the TEM at 50000-fold magnification and subjected to image processing, and equivalent circle diameters of all transition-element-dispersed particles which can be identified (can be distinguished) and whose equivalent circle diameters can be measured are measured in measurement view fields (in a total area of observation view fields of 4 μm² or more).

In this manner, there are determined the average equivalent circle diameter of transition-element-dispersed particles, and the average number density (number per cubic micrometer) of transition-element-dispersed particles each having an equivalent circle diameter of 20 to 400 nm.

The measurements of the average equivalent circle diameter and the average number density are performed on ten samples sampled from an arbitrary thickness central part. The ten measurements are averaged, and the average is defined as the average number density (number per cubic micrometer) of transition-element-dispersed particles each having an equivalent circle diameter of 20 to 400 nm.

The “transition-element-dispersed particles” specified in the present invention are identified as transition-element-dispersed particles (precipitates) containing at least one of Mn, Cr, Zr, and Sc and are distinguished from other precipitates (dispersed particles) devoid of these transition elements by observing the thickness central part with the TEM, and analyzing the view field of the thickness central part with an X-ray spectrometer through energy dispersive X-ray analysis (EDX).

In this distinguishment, the content of at least one of Mn, Cr, Zr, and Sc in a particle (precipitate) has only to be such an amount (trace amount) detectable by the EDX. A precipitate in the view field, when containing at least one of Mn, Cr, Zr, and Sc as detectable by the EDX, is defined as a “transition-element-dispersed particle” specified in the present invention, regardless of the amount of the element or elements.

As used herein, the term “equivalent circle diameter” refers to the diameter of an equivalent circle, which is determined by image-processing the images of dispersed particles identified (identifiable) by the EDX as transition-element-dispersed particles containing at least one of Mn, Cr, Zr, and Sc, calculating the area of each transition-element-dispersed particle in the TEM view field, and converting the area into the diameter of a circle having the same area as with the particle (diameter of an equivalent circle).

Production Method

Next, a method for producing the aluminum alloy sheet according to the present invention will be illustrated below.

The aluminum alloy sheet according to the present invention is produced by a production method including production steps which are, by themselves, common or known steps. In the method, aluminum alloy ingots having the 6xxx-series chemical composition are made by casting, subjected sequentially to homogenization (soaking), hot rolling, and cold rolling to have a predetermined thickness, and further subjected to heat treatments such as solution treatment and quenching, to give the aluminum alloy sheet.

However, there are preferred production conditions in these production steps, so as to give the microstructure specified in the present invention by the average number density of transition-element-dispersed particles, and the average grain size.

Melting, and Casting Cooling Rate

Initially, in the melting-casting step, a molten aluminum alloy melted and adjusted to have a chemical composition within the 6xxx-series chemical composition range is cast into ingots by a melting-casting technique. The melting-casting technique may be selected as appropriate from common melting-casting techniques such as a continuous casting technique and a semicontinuous casting technique (direct chill casting (DC casting) technique).

Homogenization (Soaking)

Next, the cast aluminum alloy ingots are subjected to homogenization before hot rolling. This homogenization (soaking) is important not only for a common purpose, i.e., homogenization of the microstructure (to eliminate or minimize grain segregation in the ingot microstructure), but also for refinement of transition-element-dispersed particles to the nanometer level, in other words, for restraining of the transition-element-dispersed particles from coarsening.

For the purpose, the soaking should be performed under conditions in which the temperature rise (heating) and cooling processes are controlled, while the soaking may be performed as any of single soaking, double soaking, and two-stage soaking.

In the double soaking, the work after first soaking is once cooled down to a temperature of 200° C. or lower including room temperature, further reheated, and held at that temperature for a predetermined time, followed by hot rolling start. In contrast, in the two-stage soaking, the work after first soaking (first-stage soaking) is cooled down to a temperature not being 200° C. or lower, but being higher than 200° C., and held at that temperature, followed by hot rolling start as intact at that temperature or after reheated to a higher temperature.

The single soaking, or first soaking in the double soaking, or first-stage soaking in the two-stage soaking is performed under conditions selected as appropriate, at a temperature in the range from 500° C. to lower than the melting point for a holding time in the range from one minute to one hour, so as to restrain transition-element-dispersed particles from coarsening and to control the particles to have a size and to be present in a number density within the specified ranges.

The heating (temperature rise) in the single soaking, or first soaking in the double soaking, or first-stage soaking in the two-stage soaking is preferably performed as rapid heating at a high rate of temperature rise of 100° C./hr or more so as to allow the transition-element-dispersed particles to precipitate as finely as possible at the nanometer level. The heating, if performed slowly at a rate of temperature rise of less than 100° C./hr, may cause the transition-element-dispersed particles to coarsen and to fail to precipitate as finely as possible at the nanometer level.

In contrast, unlike common procedures, cooling alter the first soaking in the double soaking, or cooling after the first-stage soaking in the two-stage soaking is preferably performed as slowly as possible at a low cooling rate of 40° C./hr or less, and preferably 30° C./hr or less. This may be achieved typically by performing the cooling through natural cooling, not out of the furnace, but in the furnace.

This promotes the precipitation and growth of the nanometer-level fine transition-element-dispersed particles during cooling and controls the particles to have a size within the specific range and to be present in a number density at the specific level or higher.

In the single soaking step, cooling after single soaking is also preferably performed as slowly as possible at a low cooling rate of 40° C./hr or less, as with the above cases.

The second or second-stage soaking is preferably performed at a temperature within the range from the hot rolling start temperature up to 500° C. for a holding time of 30 minutes or longer. Preferably, the ingots after the first soaking and cooling are reheated and then cooled down to the hot rolling start temperature, or reheated up to the hot rolling start temperature and held at a temperature adjacent to that temperature. Alternatively, the ingots after the first-stage soaking may be cooled down to the hot rolling start temperature and held at a temperature adjacent to that temperature. The second or second-stage soaking is performed at a lower temperature as compared with the first or first-stage soaking temperature. Also the single soaking can give similar effects when performed while controlling the time until the temperature of the work after soaking reaches the hot rolling start temperature.

The rate of temperature rise in heating to the second or second-stage soaking temperature, and the cooling rate in cooling after the soaking do not necessarily fall within the same ranges as in the first or first-stage soaking, as long as after-mentioned hot rolling conditions are met. This is because transition-element-dispersed particles in desired forms (shapes) have been formed in the first or first-stage soaking step.

Hot Rolling

The hot rolling of ingots after homogenization include a rough rolling step of ingots (slabs) and a finish rolling step performed according to the target thickness after rolling. The rough rolling step and the finish rolling step may be performed appropriately using a rolling mill such as a reverse mill or a tandem mill.

The hot rough rolling start temperature, which is the hot rolling start temperature, is preferably from 350° C. up to the solidus temperature in the single soaking step; and is preferably from 350° C. to 400° C. in the double soaking step. The hot rough rolling, if started at a temperature lower than 350° C., may hardly hot-roll the work in any soaking step. In contrast, the hot rough rolling, if started at a temperature higher than 400° C., may cause transition-element-dispersed particles to precipitate coarsely in the work from the double soaking step and may highly possibly fail to allow the transition-element-dispersed particles to precipitate as finely as possible at the nanometer level. In the single soaking step, the work is subjected soaking for a time within the predetermined range and then subjected to hot rolling without delay. This allows hot rolling to be performed while restraining the transition-element-dispersed particles from coarsening and maintaining the particles in the desired forms (shapes).

After the hot rough rolling as above, hot finish rolling is performed preferably to an end temperature in the range from 300° C. to 350° C. The hot finish rolling, if performed to an excessively low end temperature lower than 300° C., may cause deterioration in productivity due to a high rolling load. In contrast, assume that the hot finish rolling is performed to a higher end temperature so as to reduce the amount of a deformed microstructure remained in the microstructure and to allow the work to include a recrystallized microstructure. In this case, the hot finish rolling, if performed to an end temperature higher than 350° C., may cause the transition-element-dispersed particles to precipitate coarsely, and may highly possibly fail to allow the particles to precipitate as finely as possible at the nanometer level.

Heat Treatment of Hot Rolled Sheet

The hot-rolled sheet may be subjected to a heat treatment (annealing) before cold rolling, not necessarily, but optionally.

Cold Rolling

In the cold rolling, the hot-rolled sheet is rolled into a cold-rolled sheet (including one in the form of a coil) having a desired final thickness. To further refine grains, the cold rolling is desirably performed to a cold rolling reduction of 30% or more. Process annealing may be performed between cold rolling passes for the same purpose as with the heat treatment (annealing).

Solution Treatment and Quenching

After the cold rolling, solution treatment and subsequent quenching down to room temperature are performed.

The solution treatment and quenching may be performed using a common continuous heat treatment line.

However, to sufficiently solutionize elements such as Mg and Si, it is preferred that the work is subjected to solution treatment at a temperature in the range from 550° C. up to the melting temperature, and then cooled down to room temperature at an average cooling rate of 20° C./second or more. The solution treatment, if performed at a temperature lower than 550° C., causes insufficient reversion of Mg—Si compounds and other compounds which have been formed before the solution treatment. This reduces the amounts of solute Mg and solute Si.

The cooling, if performed at an average cooling rate of less than 20° C./second, causes precipitates mainly including Mg—Si precipitates to form during cooling, and this reduces the amounts of solute Mg and solute Si. Thus, the aluminum alloy sheet may highly possibly fail to include sufficient amounts of solute Si and solute Mg. To achieve the above-mentioned cooling rate in the quenching, cooling conditions and a cooling means are appropriately selected. For example, the cooling means is selected typically from a fan and other air cooling means; mist, spraying, immersion, and other water cooling means.

Pre-Aging: Reheat Treatment

After the solution treatment and cooling down to room temperature as above, the cold-rolled sheet is preferably subjected to pre-aging (reheat treatment) within one hour. Assume that the sheet is held at room temperature for an excessively long time between the completion of quenching down to room temperature and pre-aging start (heating start). The holding in this rose may cause Si-rich Mg—Si clusters to form due to natural aging at room temperature and may impede the formation and increase of Mg—Si clusters having good balance between Mg and Si. To eliminate or minimize this, the holding time at room temperature is preferably minimized, and the solution treatment-quenching and the reheat treatment may be performed successively with approximately no time lag. The lower limit of the holding time is not particularly determined.

In the pre-aging, the work is preferably held at 60° C. to 120° C. for a holding time of 5 hours to 40 hours. This allows the formation of Mg—Si clusters having good balance between Mg and Si.

The pre-aging, if performed at a temperature lower than 60° C., or for a holding time shorter than 10 hours, tends to hardly restrain the Si-rich Mg—Si clusters from forming and to hardly increase the Mg—Si clusters having good balance between Mg and Si, and tends to cause the work to have low yield strength after baking finish, as with the case where no pre-aging is performed.

In contrast, the pre-aging, if performed at a temperature higher than 120° C., or for a holding time longer than 40 hours, causes the formation of excessively large amounts of precipitation nuclei, thereby causes the work to have excessively high strength upon bend forming before baking finish and to tend to have inferior bendability.

The present invention will be illustrated in further detail with reference to several experimental examples below. It should be noted, however, that the examples are by no means intended to limit the scope of the invention; that various changes and modifications can naturally be made therein without deviating from the spirit and scope of the invention as described herein; and that all such changes and modifications should be considered to be within the scope of the invention.

EXAMPLES

As examples (experimental examples) of the present invention, 6xxx-series aluminum alloy sheets differing in average number density of the nanometer-level fine transition-element-dispersed particles and/or the average grain size were individually produced by changing chemical compositions and/or production conditions. The bake hardenability (paint bake hardenability) of the sheets after production was measured and evaluated. Results of these measurements and evaluations are given in Tables 1, 2, and 3.

The 6xxx-series aluminum alloy sheets were individually produced in the following manner. Specifically, 6xxx-series aluminum alloy sheets having the chemical compositions given in Table 1 were individually produced by changing conditions such as soaking conditions and hot rough rolling start temperature, as given in Tables 2 and 3.

A blank field in an element content in Table 1 indicates that the content of the element is equal to or lower than the detection limit (including 0%).

Specifically, the aluminum alloy sheets were produced under conditions as follows. Aluminum alloy ingots having the chemical compositions given in Table 1 were made in common by melting and DC casting. Next, the ingots were subjected to soaking in common to each sample, but under different conditions as given in Tables 2 and 3, followed by hot rough rolling started at the different temperatures given in Tables 2 and 3.

The works were then subjected to hot finish rolling in common to each sample to an end temperature in the range from 300° C. to 350° C. and rolled into hot-rolled sheets having a thickness of 4.0 mm in common. The hot-rolled sheets were, in common to each sample, subjected to cold rolling at a reduction ratio of 50% without rough rolling (heat treatment) after hot rolling and without process annealing between cold rolling passes, and thereby rolled into cold-rolled sheets having a thickness of 2.0 mm.

The cold-rolled sheets were further subjected to heat treatment (T4) using heat treatment equipment under common conditions to each sample. Specifically, the works were subjected to solution treatment at 550° C. for 30 minutes, in which the works were heated up to the solution treatment temperature at an average rate of temperature rise of 10° C./second. After the solution treatment, the works were cooled down to room temperature by water cooling at an average cooling rate of 100° C./second. Immediately after cooling, the works were pre-aged by holding at 100° C. for 8 hours, and thereafter slowly cooled (naturally cooled).

The works were left stand at room temperature for 2 weeks after these heat treatments to give final product sheets, from which test sample sheets (blanks) were cut out. Of the test sample sheets, microstructural properties were measured by the above-mentioned measurement method, where the microstructural properties include the average equivalent circle diameter (nm) of transition-element-dispersed particles, the average number density (number per cubic micrometer) transition-element-dispersed particles each having an equivalent circle diameter of 20 to 400 nm, and the average grain size (μm).

The bake hardenability of the test sample sheets was measured and evaluated. Results of these measurements and evaluations are given in Tables 2 and 3.

Bake Hardenability

Of the test sample sheets, an As 0.2% yield strength, a 0.2% yield strength after 2% stretching, and a 0.2% yield strength after BH were determined by tensile tests. The As 0.2% yield strength is the yield strength of a T4 sheet before forming (shaping) and bake hardening (BH). The 0.2% yield strength after 2% stretching is the yield strength of the sheet after 2% stretching. The 0.2% yield strength after BH is the yield strength of the sheet after 2% stretching and subsequent artificial aging.

The 2% stretching simulated bend forming as forming of the material sheet into a structural component. The artificial aging (BH) was performed at 185° C. for 20 minutes.

Tables 2 and 3 sequentially present the As 0.2% yield strength, an increase in 0.2% yield strength after the 2% stretching, and the 0.2% yield strength after BH.

The tensile tests were performed at room temperature on JIS Z 2201 No. 5 test specimens (25 mm by 50 mm GL by thickness) sampled from the test sample sheets. In the tests, the test specimens were pulled in a direction parallel to the sheet rolling direction. The tests were performed at a gauge length of 50 mm and at a constant tensile speed of 5 nm/min until the test specimens ruptured. These mechanical properties were measurd on five test specimens (N=5), and averages of the five measurements were defined as, and evaluated as, the mechanical properties.

Tables 1 and 2 demonstrate as follows. Examples 1 to 13 had chemical compositions within the ranges specified in the present invention, and were produced under conditions within the preferred ranges. The examples therefore had microstructures as specified in the present invention, as demonstrated in Table 2, where the microstructures are specified by the average equivalent circle diameter of transition-element-dispersed particles, the average number density of transition-element-dispersed particles each having an equivalent circle diameter of 20 to 400 nm, and the average grain size.

As a result, the examples each had a high 0.2% yield strength after BH even after natural aging at room temperature as demonstrated in Table 2 and were found to have high strength Specifically, the examples had a large increase in 0.2% yield strength of 33 MPa or more after 2% stretching and were found to have better work hardenability upon forming of the material sheet into an automobile structural component.

In addition, the examples each had a high yield strength (strength) upon artificial aging of the automobile structural component after forming, and, as a result of the synergistic effects with the above-mentioned effect during stretching (forming), the examples had significantly better bake hardenability of 262 MPa or more.

In contrast, Comparative Examples 14 to 21 in Table 3 employed Alloy No. 1 or 4 in Table 1, as with the examples. These comparative examples, however, were produced under conditions out of any of the preferred ranges as given in Table 3, where the conditions include soaking conditions and hot rough rolling start temperature.

Comparative Example 14 underwent soaking performed at an excessively low temperature of lower than 500° C.

Comparative Examples 15, 18, and 21 underwent excessively slow heating in soaking, at a rate of temperature rise of less than 100° C./hr.

Comparative Example 19 underwent excessively rapid cooling after soaking, at a cooling rate of greater than 40° C./hr.

Comparative Examples 15, 16, and 18 underwent a double soaking step, but underwent hot rough rolling started at an excessively high temperature of higher than 400° C.

Comparative Examples 17 and 20 underwent a double soaking step, but underwent hot rough rolling started at an excessively low temperature of lower than 350° C.

These comparative examples were therefore had microstructures not meeting one or more of the conditions specified in the present invention, where the conditions include the average number density of transition-element-dispersed particles each having an equivalent circle diameter of 20 to 400 nm, and the average grain size.

As a result, the comparative examples had a small increase in 0.2% yield strength of at most about 30 MPa after 2% stretching, failed to offer the synergistic effects with improvement in yield strength upon BH, and had a poor bake hardenability of at highest about 248 MPa after natural aging at room temperature, as compared with the examples having the same alloy chemical compositions.

Comparative Examples 22 to 27 in Table 3 were produced under conditions within the preferred ranges, but employed Alloy Nos. 9 to 14 in Table 1, where the alloys have chemical compositions out of any of the ranges specified in the present invention.

Comparative Examples 22 and 23 respectively employed Alloy Nos. 9 and 10 in Table 1 and had an excessively low total content of at least one of Mn, Zr, Cr, and Sc.

Comparative Example 24 employed Alloy No. 11 in Table 1 and had an excessively low Mg content.

Comparative Example 25 employed Alloy No. 12 in Table 1 and had an excessively high Mg content.

Comparative Example 26 employed Alloy No. 13 in Table 1 and had an excessively low Si content.

Comparative Example 27 employed Alloy No. 14 in Table 1 and had an excessively high Si content.

As demonstrated in Table 3, these comparative examples therefore had microstructures not meeting one or more of the conditions specified in the present invention, where the conditions include the average number density of transition-element-dispersed particles each having an equivalent circle diameter of 20 to 400 nm, and the average grain size.

As a result, the comparative examples had a small increase in 0.2% yield strength of at most about 30 MPa after 2% stretching, failed to offer the synergistic effects with improvement in yield strength upon BH, and had a poor bake hardenability of at highest about 244 MPa after natural aging at room temperature.

The results of these experimental examples demonstrate that aluminum alloy sheets have to meet all the conditions in chemical composition and microstructure specified in the present invention, so as to have higher strength even after natural aging at room temperature.

TABLE 1 Al—Mg—Si aluminum alloy sheet chemical composition Alloy (in mass percent; reminder: Al) number Mg Si Fe Mn Zr Cr Sc Cu Ag Sn Ti 1 0.8 1.0 0.14 0.46 0.04 0.02 2 0.8 1.0 0.14 0.45 0.15 0.04 0.02 3 0.7 0.9 0.15 0.05 0.02 4 1.2 1.0 0.34 0.07 0.40 0.02 5 1.5 0.7 0.14 0.45 0.06 0.15 0.06 6 1.1 1.4 0.14 0.70 0.06 0.05 0.05 0.03 7 0.4 1.0 0.14 0.15 0.40 0.14 0.02 8 0.8 1.0 0.13 0.15 0.15 0.03 0.01 0.02 9 0.8 1.0 0.14 0.08 0.02 10 0.8 1.0 0.14 0.03 0.02 11 0.2 0.8 0.08 0.02 12 1.6 1.0 0.15 0.04 0.20 0.02 13 1.5 0.2 0.14 0.15 0.06 0.08 0.02 14 1.1 1.6 0.14 0.10 0.05 0.04 0.02

TABLE 2 Aluminum alloy sheet production conditions Hotrough Alloy First soaking Second soaking roling number Rate of Soaking Cooling Soaking Start in Soaking temperature temperature Holding rate temperature Holding temperature Category Number Table 1 pattern rise (° C./hr) (° C.) time (hr) (° C./hr) (° C.) time (hr) (° C.) Examples 1 1 double 100 530 1 40 400 2 400 2 1 double 150 520 0.6 35 390 4 390 3 1 double 120 510 1 40 450 0.5 400 4 1 single 100 550 0.4 5 — — 550 5 1 single 150 540 1 2 — — 520 6 1 single 120 560 0.3 3 — — 560 7 2 double 140 530 1 25 400 2 390 8 3 double 130 510 0.8 35 390 1 380 9 4 double 100 505 0.9 40 430 3 400 10 5 double 110 530 1 25 375 8 375 11 6 double 160 535 0.5 30 395 2 395 12 7 double 120 515 0.6 40 380 4 370 13 8 double 150 520 0.8 40 410 3 400 Aluminum alloy sheet after left stand at room temperature for 2 weeks Microstructure Properties Average number Increase in Average equivalent density of As 0.2% 0.2% yield 0.2% Yield circle diameter of transition-element-dispersed Average yield strength after strength transition-element-dispersed particles grain strength 2% stretching after BH Category Number particles (nm) (number/μm³) size (μm) (MPa) (MPa) (MPa) Examples 1 137 12.4 58 195 37 317 2 132 13.3 49 199 39 319 3 105 12.8 54 198 38 306 4 261 10.2 71 192 35 314 5 215 11.1 65 191 36 311 6 297 8.7 82 193 33 316 7 103 26.8 41 194 41 327 8 89 5.5 73 207 34 306 9 96 5.1 86 168 33 262 10 121 16.7 43 201 40 323 11 187 38.1 43 215 45 330 12 102 5.3 78 210 35 298 13 116 13.2 69 194 39 301

TABLE 3 Aluminum alloy sheet production conditions Hotrough Alloy First (first-stage) soaking Second soaking roling number Rate of Soaking Cooling Soaking Start in Soaking temperature temperature Holding rate temperature Holding temperature Category Number Table 1 pattern rise (° C./hr) (° C.) time (hr) (° C./hr) (° C.) time (hr) (° C.) Comparative 14 1 single 150 480 1 10 — — 470 Example 15 1 double 20 570 1 15 500 8 470 16 1 double 100 500 1 40 480 2 460 17 1 double 100 500 1 30 350 10  330 18 4 double 80 500 1 20 420 6 420 19 4 double 100 500 1 70 360 4 360 20 4 double 100 500 1 40 330 2 330 21 1 single 15 500 2 5 — — 490 22 9 double 150 500 1 15 400 4 390 23 10 double 150 500 1 15 400 4 390 24 11 double 150 500 1 15 400 4 390 25 12 double 150 500 1 15 400 4 390 26 13 double 150 500 1 15 400 4 390 27 14 double 150 500 1 15 400 4 390 Aluminum alloy sheet after left stand at room temperature for 2 weeks Microstructure Properties Average number Increase in Average equivalent density of As 0.2% 0.2% yield 0.2% Yield circle diameter of transition-element-dispersed Average yield strength after strength transition-element-dispersed particles grain size strength 2% stretching after BH Category Number particles (nm) (number/μm³) (μm) (MPa) (MPa) (MPa) Comparative 14 243 3.1 62 204 28 248 Example 15 195 4.7 56 193 29 237 16 93 4.4 58 182 30 241 17 88 4.2 75 190 31 248 18 101 4.2 82 187 27 239 19 90 4.3 73 189 28 237 20 87 3.8 80 202 29 247 21 92 4.3 102 187 27 241 22 131 2.7 153 188 29 228 23 120 2.1 137 191 28 229 24 146 2.9 95 142 29 193 25 121 3.8 183 177 28 212 26 128 4.1 88 138 30 174 27 117 4.0 74 196 30 244

While the present invention has been particularly described with reference to specific embodiments thereof it is obvious to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the present invention.

This application claims priority to Japanese Patent Application No. 2015-108596, filed on May 28, 2015, the entire contents of which are incorporated herein by reference.

INDUSTRIAL APPLICABILITY

The present invention can provide 6xxx-series aluminum alloy sheets having higher strength. This enlarges the applications of such 6xxx-series aluminum alloy sheets further to, in addition to panel components, automobile structural components exemplified typically by frame components such as frames and pillars; and reinforcements such as bumper reinforcements and door beams. 

1. A high-strength aluminum alloy sheet being an Al—Mg—Si aluminum alloy sheet comprising, in mass percent: Mg in a content of 0.3% to 1.5%; Si in a content of 0.3% to 1.5%; at least one transition element selected from the group consisting of: Mn in a content of 0.1% to 0.8%; Zr in a content of 0.04% to 0.20%; Cr in a content of 0.04% to 0.20%; and Sc in a content of 0.02% to 0.1%; and Al and unavoidable impurities, wherein, in a microstructure in a thickness central part of the sheet: the sheet has an average grain size of 100 μm or less; dispersed particles comprising the at least one transition element have an average equivalent circle diameter of 50 to 300 nm, as measured by TEM-EDX at 50000-fold magnification; and transition-element-dispersed particles each having an equivalent circle diameter of 20 to 400 nm are present in an average number density of 5 or more per cubic micrometer.
 2. The high-strength aluminum alloy sheet according to claim 1, further comprising: at least one element selected from the group consisting of: Cu in a content of greater than 0% to 0.5%; Ag in a content of 0.01% to 0.2%; and Sn in a content of 0.001% to 0.1%. 